About This Article
This article was created using an automated generation workflow powered by generative AI. We use fictitious data and browser operations to learn the use cases, mechanisms, and observation points for failures when using QRCode.js. No access to real services or personal information is performed.Verification Status:Official documentation and HTML/JavaScript from the pinned GitHub commit were compared on 2026-10-30. Actual browser operations, CDN fetching, and rendering/audio output were not executed, and the results in the text represent expected values based on the code.
| Item | Details |
|---|---|
| Target | Modern browsers such as Chrome, Edge, and Firefox (actual device compatibility unverified) |
| Library Used | QRCode.js (Tutorial version: 1.0.0) |
| Cost | No paid API is required for the demo source itself. Communication fees and usage conditions depend on the environment. |
| Data | Fictitious data within the page. No authentication info, location data, or corporate data required. |
| Learning Operation | Converting a fictitious event name into a QR code |
- Why Test This Technology?
- Try it first: Live demo and expected results
- How It Works: Decoding the API
- Comparing by Changing Only One Condition
- Where to Look When Failures Occur
- Application to Business and Learning
- Comparing with Alternative Methods
- GitHub sample
- Official and primary sources
- Supplementary Information on Verification Scope
Why Test This Technology?
QR codes are increasingly used on event notices and guide cards. While QR codes are often thought of as a way to embed URLs, they are actually a mechanism to represent data such as arbitrary strings. Using QRCode.js, you can generate small QR codes within a browser page. In this exercise, we will not use real application sites or personal contact info, but rather only a fictitious event name and the string 'Date and time to be announced'. Even if a QR code is convenient, whether the link destination is correct and whether the content is safe to publish are entirely separate issues.
When building a working sample, do not just stop at the fact that the visual interface updated; you must distinguish between which input modified which variable and which process updated the rendering. In this learning material, those changes are explicitly displayed on the screen. When integrating into an actual service, please design the authentication and storage processes separately.
Try it first: Live demo and expected results
Change the initial "Stargazing Event" to another fictional name and click the generate button. The screen will display not only the QR pattern but also the actually stored string in plain text side by side. If you have a scanning app, you can test whether both match. However, since scanning devices and apps may use external services, do not enter any personal information. Another observation point is that inputting an empty field displays an error, preventing an old QR code from being treated as successful with new content.
Things to check before operating
Confirm that your PC can execute JavaScript. Administrator privileges are not required.
Open the browser's developer tools and prepare to check the Console for errors and the Network tab for any CDN retrieval failures.
If the demo is not actually published, save the Daily Code
index.htmlandapp.jsin the same folder and open them using a local HTTP server. When using modules, direct execution via file:// is not guaranteed to work.Compare the results before and after the operation. If they differ from the expected values, record "what was not displayed" first.
Overview of the mechanism
flowchart LR
A["入力やクリック"] --> B["イベントを受ける"]
B --> C["QRCode.js のAPI"]
C --> D["状態と描画を更新"]
D --> E["見た目と文字で確認"]
This diagram is a conceptual illustration for learning purposes and does not represent the internal implementation of the browser or the details of network communication. Regardless of the library, tracing "which variable receives the operation" remains consistent.
Local verification when the public demo is unavailable
Save the pinned commitHTML fileandJavaScript filefrom the GitHub Raw view, respectivelyindex.htmlandapp.jssave them in the same new folder. The README alone will not make it work. On Windows, open PowerShell in that folderpy -3 -m http.server 8000 --bind 127.0.0.1, and on Linux/macOSpython3 -m http.server 8000 --bind 127.0.0.1, run the command and openhttp://127.0.0.1:8000/on the same PC. To exit, pressCtrl+Cin the terminal. Because the browser fetches the library core from an external CDN, check the Console and Network errors if a connection cannot be established. This procedure is a reproduction guide and not a browser measurement report for this article.
How It Works: Decoding the API
Pass the destination DOM element, text, width and height size, and error correction level to the QRCode constructor. The library converts the string into a QR code pattern and renders it as a canvas or HTML element. If you keep increasing QRCode instances every time the input changes, previous images may remain or the DOM may bloat. The demo retains the existing instance and rewrites it using clear and makeCode. When the string changes, the pattern changes as well, but the content cannot be read from appearance alone.
How to Read the Minimal Code
const qrcode = new QRCode(document.getElementById("qrcode"), {
text: "架空イベント：星空観察会", width: 180, height: 180
});
qrcode.makeCode("別の架空イベント");
The above is an excerpt highlighting the main points of the API. It is not a complete file meant to be copied and run standalone.Please check the README/demo files on the Daily Code sidefor the HTML and complete implementation required to run it. Trace which function selects the HTML element and which line updates the numerical values, and test with known dummy values.
Comparing by Changing Only One Condition
Packing a long sentence into a QR code makes the pattern finer, which can make it difficult to read when printed. When embedding a destination URL, verify the domain, HTTPS, and expiration date in advance. If the site accessed via the QR code changes, the content of a statically printed QR code does not change. For frequently changing information, setting up a modifiable landing page is often easier to maintain.
The comparison procedure is to record the initial value, change only one part, and verify whether the result appears in the expected location. If you change multiple inputs at once, it becomes impossible to know what had an effect. Make sure to include the operation of reverting to the pre-change state in your verification.
Where to Look When Failures Occur
QR codes are not encryption or authentication. Because scanning reveals the string, it is important not to easily include passwords, secret tokens, personally identifiable information, or private meeting URLs. Furthermore, even if the QR code itself is not malicious, it may direct scanned users to a fake site. Display the published domain in plain text so it can be verified visually, and consider regular link checks during operation.
| Observed State | Points to Isolate | Example Action |
|---|---|---|
| The screen remains in its initial state | Script acquisition, initialization, and event registration | Check Console, Network, and script tags |
| The display changes, but the meaning is incorrect | Input values, arrays, units, and update targets | Fix the value and check one condition at a time |
| The button cannot be clicked | Selectors and DOM generation order | Check the HTML to see if the IDs match |
| It only fails on certain devices | Browser features, permissions, CDN, and performance | Record the device and version along with reproduction steps |
Errors within a browser are distinct from outages on a public web server. The former should be investigated using developer tool error logs, while the latter should be checked through HTTP responses and asset delivery conditions. Refraining from labeling an unknown failure as a success enhances the reliability of technical notes.
Application to Business and Learning
This can be used for information cards, exhibition commentary pages, and links to public equipment manuals. When applying this to internal business procedures, ensure you design who can modify URLs, how printed materials will be recalled, and how to handle revocation when necessary. Even though code generation itself is straightforward, managing the lifespan of links is critical.
Decisions to Make Before Implementation
When publishing content to readers, determine where the displayed data originates, who can modify it, what types of information remain on the screen, and the verification items required when updating libraries. This demo includes neither login nor server-side storage. Therefore, before adding operational features, we recommend first distinguishing between the 'minimum viable portion that displays as expected' and the 'portion requiring operational reliability, permissions, and backups.' Because third-party CDNs rely on external communication, verify in advance whether they are permitted on your corporate network.
Comparing with Alternative Methods
| Method | Suitable Scenario | Points to note |
|---|---|---|
| QRCode.js | Suitable scenarios such as converting a fictional event name into a QR code | Requires updating dependency libraries and managing usage conditions |
| Browser standard API | Build small features in-house | Check specifications and compatibility yourself |
| Static images and tables | Explanations that do not require interaction | State changes cannot be experienced on the spot |
Libraries are tools to be chosen when needed. Compare what you want to achieve against the learning cost. Avoiding making "adding a library" the main goal makes smaller pages easier to maintain.
GitHub sample
The blog text focuses on background and concepts, while Daily Code provides code for execution and modification, separating their roles. APIs for posting or saving to external services are not included.
Official and primary sources
Scope of reference verification:Refer to the official documentation as of the edit date of October 27, 2026. This does not guarantee future changes to the linked destinations or all CDN asset responses.
Supplementary Information on Verification Scope
The fixed sources stored on GitHub were cross-checked against the official API documentation. Browser operations, CDN loading, and results on each OS have not been verified. When confirming actual behavior, check the Console and Network, and do not conclude that all conditions were successful based solely on the SUCCESS display on the screen.
